xen/arm32: head: Fix build when using GAS 2.25.0

xen/arm32: head: Fix build when using GAS 2.25.0

GAS 2.25.0 throws multiple errors when building arm32/head.S:

arm32/head.S: Assembler messages:
arm32/head.S:452: Error: invalid constant (f7f) after fixup
arm32/head.S:453: Error: invalid constant (f7f) after fixup
arm32/head.S:495: Error: invalid constant (f7f) after fixup
arm32/head.S:510: Error: invalid constant (f7f) after fixup
arm32/head.S:514: Error: invalid constant (f7f) after fixup
arm32/head.S:516: Error: invalid constant (f7f) after fixup
arm32/head.S:633: Error: invalid constant (f7f) after fixup

This makes sense because the instruction mov is only able to deal with a
specific set of immediate (see "modified immediate constants in ARM
instructions"). For any 16-bit immediate, the instruction movw should be
used.

It looks like newer version of GAS will seemly switch to movw if the
immediate does not fit in the immediate encoding for mov. But we should
not rely on this. So switch to movw.

Fixes: 23dfe48d10 ("xen/arm32: head: Introduce macros to create table and mapping entry")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>